This invention pertains generally to the field of radioactive power sources and to micromechanical and microelectromechanical devices.
Manufacturing techniques have been developed for the production of microelectromechanical systems (MEMS) using manufacturing technologies adapted from the manufacturing of integrated circuits and other electronic devices. Such MEMS structures may be formed of a variety of materials, including semiconductors used in integrated circuit manufacturing, such as silicon, and various metals. The small size of MEMS systems and the materials of which they are formed naturally offers the opportunity for the integration of these structures with integrated circuits to provide totally autonomous Microsystems. However, such systems still require a power source, and the utility of autonomous Microsystems has been hindered by the unavailability of suitable power sources. Even the smallest conventional batteries may be much larger than the MEMS system being supplied with power, thus limiting the extent to which the size of the overall device can be shrunk. In addition, conventional batteries have a relatively short useful lifetime, typically on the order of days to weeks or at most months, whereas in some applications it would be desirable to have a power source capable of supplying power to the MEMS device for many months or even years. Suitable devices with sufficient lifetime would be useful for a variety of applications. For example, sensor systems placed over a large area may be utilized to monitor vibration and gas output of vehicles and report back the information to a central collection point via optical or radio frequency (RF) communications. The signals produced by the small autonomous sensors may be picked up and stored and amplified by a larger central system powered by conventional sources such as gasoline engines, fuel cells, or large batteries. Such sensors have also been proposed for use in battlefield monitoring and in commercial applications for sensing properties that affect component life such as viscosity, Young""s modulus, vibration, etc. If such devices could be provided with power sources capable for operating for years or decades without replacement, the sensors could be embedded inside permanent casings such as walls of buildings, or could be utilized in space research as xe2x80x9cmicrosatellites.xe2x80x9d
One proposed approach to providing long-lived power sources is the use of radioisotopes that generate electrical power in a nuclear xe2x80x9cbattery.xe2x80x9d Early approaches to such batteries are discussed in A. Thomas, xe2x80x9cNuclear Batteries: Types and Possible Uses,xe2x80x9d Nucleonics, Vol. 13, No. 11, November 1955. One approach to electric power generation from radioisotopes is based on charge particle collection. See, e.g., G. H. Miley, xe2x80x9cDirect Conversion of Nuclear Radiation Energy,xe2x80x9d American Nuclear Society, 1970; L. C. Olsen, et al., xe2x80x9cBetavoltaic Nuclear Electric Power Sources,xe2x80x9d Winter Meeting of the American Nuclear Society, San Francisco, Calif., 1969. Most of the nuclear battery designs are based on thermal effects, in which a volume of the source is self-heated due to highly energetic particle impacts, and the heat energy is then converted to electrical energy, with a typical efficiency of 4% to 15% (a quantity determined by the efficiency of the thermo-electric converter). Although such an approach is amenable to miniaturization, the need for thermal isolation and relatively high operating temperatures makes such devices suitable primarily where high power is required and where the high operating temperature and volume of heat produced is not problematic. Further, as devices shrink in size, the surface to volume ratio increases, with large losses from radiative and convective losses. In particular, the stopping depth for electrons or alpha particles in materials is usually in the range of a few microns to several tens of microns, indicating that the charged particle collector must be at least that many microns thick. This implies that the collector cannot be scaled down to a thickness less than the decay depth since otherwise the emitted particles simply pass through. Consequently, it is unlikely that traditional thermal conversion will work in microscale devices. Another approach for converting emitted charged particles to electric power is by the creation of electron-hole pairs by ionization in a semiconductor (e.g., silicon). In a depletion region electric field, the pairs can be separated to provide electric energy. This is essentially the same principle used in solar cells, where photons cause electron-hole pair generation. An advantage of the use of particles from nuclear decay is that they create thousands of electron-hole pairs per emitted particle because of the large particle energy. However, a significant disadvantage is that the high energy of the particles damages the crystal lattice, which in turn reduces the effectiveness of the capture of more particles. Although there are ways to continuously or intermittently thermally anneal the crystal, it is unlikely that such annealing will result in a fully repaired crystal and it is a process that is difficult to utilize in devices that are in place in the field. Furthermore, because such sources depend on the use of pn-junctions, the operating temperature range of the devices is limited to about xe2x88x9215xc2x0 C. to 100xc2x0 C. Another approach is to generate light by the incidence of the emitted particles onto a luminescent material, and then capture the emitted light with a photocell to produce electricity. However, such an approach requires very high radioactive source levels due to the low efficiency of the incident particle to photon production.
Another approach which has been considered is the use of direct charge in which charged particles, e.g., electrons, emitted from a source are collected by a collector spaced by a gap from the source, thereby building up a potential difference between the source and the collector. By increasing the gap between the source and the collector, it is theoretically possible to obtain very high voltage differences (e.g., millions of volts) due to the low capacitance between the source and the collector, but any attempt to use the power from the system to drive even a picofarad (pF) load will effectively reduce that output voltage to millivolts. Consequently, such an approach would only provide useful output voltages if the load capacitance is of the same order of magnitude as that of the source-collector capacitor. For sources having relatively low radiation flux, as generally would be the case for devices to be used in the microsensor field, the voltages that could be obtained by this approach would necessarily be quite low.
The present invention carries out direct conversion of radioactive emissions to mechanical motion such as in micromachined elements. The motion of the micromachined elements may be utilized directly to actuate other mechanical parts or the motion of the mechanical elements may be converted to electrical energy to provide a long-lived source of electrical power for microelectromechanical systems. Such electrical power generators have a very high energy density as compared to conventional power sources for microsystems, such as batteries, can be designed to have extremely long life, in the range of several years, and can provide output voltages at levels suitable for driving conventional integrated circuit electronics.
An activator in accordance with the invention is formed with a base or substrate on which is mounted an elastically deformable element that has a section that is free to be displaced toward the base. The deformable element may be formed of various types of structures that may be produced by conventional machining or by micromechanical processing, including cantilevers, bridges and deformable membranes. An absorber of radioactively emitted particles (e.g., electrons) is formed on one of the base or the displaceable section of the deformable element, and a material comprising a radioactive source is formed on the other of the base or displaceable section, facing the absorber across a gap chosen to provide a selected efficiency of particle collection. The radioactive source emits charged particles, such as electrons or alpha particles, resulting in a buildup of opposite charges on the source and absorber, thereby producing an electrostatic force between these two elements and resulting in the section of the deformable element to which the source or absorber is attached being drawn toward the base to bend the deformable element. Mechanical energy is thus stored in the elastically deformable element which is released when the absorber is drawn into effective electrical contact with the source. The elastic return of the deformable element toward its initial position releases mechanical energy which can be used to activate other mechanical elements or to generate electrical power. In a preferred micromechanical power generator, an electrical generator is coupled to the deformable element, for example, a piezoelectric element mounted to the deformable element to deform with it. As the deformable element elastically springs back toward its initial position, strain on the piezoelectric transducer is also released, resulting in electrical power generated by the piezoelectric element that may be connected from its output terminals to a load, such as a radio frequency coil. The capacitance of the piezoelectric transducer element connected to the coil provides a resonant tank circuit that produces an electrical oscillation at a characteristic frequency which is excited by the pulse of output voltage from the piezoelectric transducer. This voltage may be rectified and stored on a storage capacitor for use by other electronic components, and the high frequency oscillation may also be utilized to provide a radio signal that can be detected by remote detectors.
A preferred micromechanical generator includes a cantilever beam mounted by a post at one end to extend out over a top surface of the base, the absorber or source being formed on the free end of the beam. The beam itself may be formed of various materials commonly utilized in micromechanical systems, including single crystal silicon and polysilicon, and metals such as nickel, gold and copper. The absorber is preferably formed of a material such as a metal which can readily absorb and retain the charge of the radiated particles without long-term damage or other effect. The radioactive material forming the source preferably emits primarily beta particles (electrons) and is selected to provide a desired balance between emission activity and safety. A particularly preferred radio isotope is nickel-63, which combines the characteristics of long life (half life of 100 years), pure beta particle emission, and moderate activity levels.
An electromagnetic generator of the invention may be utilized as a sensor to provide an output signal to a remote source indicative of the quantity being sensed. For example, a piezoresistor may be connected electrically between the absorber and the source to effectively control the rate at which the absorber charges to the point of contact between source and absorber, thereby varying the period of the cycle of charging and touchdown between the source and absorber in relation to the quantity being sensed.
Further objects features and advantages of the invention will be apparent from the following detailed description when taken in conjunction with the accompanying drawings.